1. Field of the Invention
The present invention relates generally to a method of manufacturing a spindle motor for rotating a disk storage medium and, more particularly, to a method of manufacturing a spindle motor for enhancing a rotational precision of the disk storage medium.
2. Description of the Related Art
Over the recent years, an increase in storage density has been demanded of a disk storage device. For increasing this storage density, a magnetic disk apparatus is required to have a higher track density on a magnetic disk. When increasing the track density on the disk medium, however, rotational shaking of the disk medium exerts an influence on a positioning accuracy of a head. This being the case, a spindle motor demanded is the one causing a lesser quantity of rotational shaking of the magnetic disk medium.
FIG. 14 is an explanatory diagram illustrating a magnetic disk apparatus.
As shown in FIG. 14, a plurality of magnetic disks 2 are supported on a spindle motor 1. The spindle motor 1 rotates a magnetic disk 2. A magnetic head 4 is provided for reading and writing data from and to the magnetic disk 2. An actuator 3 positions the magnetic head 4 on a track on the magnetic disk 2.
In the magnetic disk apparatus, as known well, the actuator 3 positions the magnetic head 4 on a desired track on the magnetic disk 2. Then, the magnetic head 4 reads or writes the data from or to the same track.
FIG. 15 is a sectional view showing a prior art spindle motor.
A lower end of a fixed shaft 82 is secured to a base 81 of the magnetic disk apparatus. The base 81 is provided with a core 89 on which a coil 88 is wound.
A sleeve 83 is fitted to the fixed shaft 82 through a pair of ball bearings 85. The sleeve 83 is rotatable with respect to the fixed shaft 82. A hub 84 is fixed to its periphery of the sleeve 83. The hub 84 has a support member 84-1 for the magnetic disk. An upper surface of the disk support member 84-1 of the hub 84 serves as a support surface for the magnetic disk. A yoke 87 and a magnet 86 are fixed to an internal surface of the hub 84.
In this spindle motor, the magnet 86 fixed to the hub 84 is disposed in a face-to-face relationship with the coil 88 fixed to the base 81. Therefore, when flowing an electric current across the coil 88, the hub 84 rotates.
FIGS. 16A and 16B are explanatory views each showing an assembling method in the prior art.
As illustrated in FIG. 16A, the fixed shaft 82 is secured to the base 81 fitted with the coil 88. Then, the sleeve 83 and the hub 84 are attached to the fixed shaft 82 through the ball bearings 85.
Thereafter, a cutting blade 90 is pressurized upon one area of the upper surface (the support surface of the magnetic disk) of the disk support member 84-1 of the hub 84. Then, the disk support surface of the disk support member 84-1 is ground by the cutting blade 90 while rotating the hub 84.
Next, as illustrated in FIG. 16B, the magnetic disk 2 is mounted on the hub 84. At this time, the magnetic disk 2 is supported by the disk support member 84-1 of the hub 84. Then, a position of the magnetic disk 2 from the base 81 is determined by a position of the support surface of the disk support member 84-1. Note that the numeral 91 designates a spacer for taking a spacing between the magnetic disks 2, and the numeral 92 represents a cap for fixing the magnetic disk 2.
After a step shown in FIG. 16B, the actuator 3 including the magnetic head 4 is secured to the base 81. A position of the magnetic head 4 is based on a position of the surface of the magnetic disk 2. It is therefore required that a height of the magnetic disk 2 from the base 81 be set as precise as a micron order.
Incidentally, an assembly error is to be caused in the assembly of the spindle motor shown in FIG. 16A. If this assembly error is produced, the height of the disk support surface of the disk support member 84-1 from the base 81 differs according to each spindle motor. Therefore, a height of the magnetic disk 2 from the base 81 is also different depending on each spindle motor. A floating quantity of the magnetic head from the magnetic disk surface is thereby varied, which might influence read/write characteristics of the magnetic head.
The height of the magnetic disk 2 from the base 81 is fixed by each spindle motor, and hence, as shown in FIG. 16A, the disk support surface of the disk support member 84-1 of the hub 84 is ground after assembling the spindle motor.
On the other hand, an enhancement of the track density of the magnetic disk is demanded. When increasing the track density, the rotational shaking of the magnetic disk exerts the influence upon the positioning precision of the magnetic head.
The ball bearing has hitherto been used as a bearing. the ball bearing tends to cause vibrations of inner and outer rings with passages of the balls. Namely, PRO (Repeatable Run Out)/NPRO (Non Repeatable Run Out) occurs, and the rotational shaking of the spindle motor is therefore inevitable.
For preventing this rotational shaking, it is examined that a fluid bearing is used as a bearing. The fluid bearing, when rotating, comes to have a pressure produced. Subsequently, the fluid bearing keeps itself afloat in the axial direction till the gravity, a magnetic force and an external pressure balance with each other. Self-aligning action works in the radial direction. With this action, it is feasible to provide rotations through a fluid lubrication with a reduced friction between the shaft 82 and the sleeve 83. When using this fluid bearing, non-contact rotations can be actualized, which enables the rotational shaking of the spindle motor to decrease.
By the way, according to the prior art grinding method described referring to FIG. 16A, one point on the hub 84 is pressurized. The spindle motor using the conventional ball bearings is high of a rigidity of the ball bearing, and therefore no inclination of the hub 84 is produced. For this reason, the disk support surface of the disk support member 84-1 can be ground flat.
The fluid bearing has, however, a rigidity that is by far smaller than in the ball bearing. Therefore, when executing the grinding process with the conventional one-point pressurization in the prior art, the hub is inclined, and the disk support surface of the disk support member is ground unflat.
Such being the case, when mounting the magnetic disk, the magnetic disk mounted never becomes flat. The mounted magnetic disk thereby comes to have a difference in height from the base between the inner side and the outer side. Accordingly, the floating quantity of the magnetic head changes on the inner and outer sides of the magnetic disk. This leads to such a problem that the read/write characteristics of the magnetic head change on the inner and outer sides of the magnetic disk.